Engine oil protects the moving parts of an engine by providing a lubricating coating to reduce friction generated by the moving parts. The ability of the engine oil to properly lubricate the moving parts of the engine is largely dependent on rheological properties of the oil, in particular the viscosity of the engine oil. In general terms, viscosity is a measure of resistance of a fluid to flow. In an engine, the oil fills the narrow spaces between the parts and clings to both moving and non-moving parts. The tendency of the oil to remain in contact with both the moving and non-moving parts creates internal frictional forces within the oil. These internal forces must be overcome before relative movement between the parts can occur. The internal forces within the oil will vary in proportion to the viscosity of the oil and will increase with increasing viscosity. Additionally, for a given blend of engine lubricating oil, the viscosity will not remain constant, but will vary as a function of temperature, becoming much more viscous in cold temperatures. The resulting increased frictional forces associated with the increased viscosity renders engine operation or “cranking” more difficult in low temperature conditions.
Known rheological test devices include devices known as “cold-cranking simulators” which are used to test engine oils at low temperatures under simulated engine starting conditions for compliance with the Society of Automotive Engineers (SAE) Standard J300. The testing of oils using these devices is governed by the American Society for Testing and Materials (ASTM) D5293 “Standard Test Method for Apparent Viscosity of Engine Oils Between −5 and −35° C. Using the Cold-Cranking Simulator.” A cold cranking simulator measures the apparent viscosity of an engine oil by measuring the resistance to rotation imposed on a rotor by a sample of oil delivered into a narrow annular space between the rotor and a non-moving stator. The cold cranking simulator therefore differs in operation from devices such as capillary viscometers which measure flow rate of a fixed volume of a fluid through an orifice. The results of testing on a sample of engine oil using a cold cranking simulator are called the “cranking viscosity” of the engine oil.
An example of a cold cranking simulator is shown in U.S. Pat. No. 4,472,963 to Gyer. A sample of oil is introduced into a narrow annular space between a rotatably supported rotor and fixed stator. A probe is located within the stator to monitor the temperature of the stator. Methanol from a cold bath is circulated through coolant conduits in the stator to cool the stator. The methanol in the cold bath is maintained at a constant predetermined temperature differential below the test temperature. The methanol is introduced into the stator through a valve which is periodically opened and closed to control flow of coolant. A control system is responsive to the temperature from the stator probe to adjust the on-to-off time of the valve thereby controlling the amount of methanol delivered to the stator. Methanol is also heated to just below boiling in a separate hot bath for circulation through the coolant passages of the stator between tests to facilitate removal of the tested sample. The heated methanol facilitates the removal of the tested sample by reducing the viscosity of the oil thereby reducing the resistance of the oil to flow.
Methanol is a flammable and highly toxic substance. The storage and handling of the methanol in the baths and in the circulating system of the cold cranking simulator of the '963 patent therefore represents a threat to health and safety. The safety concerns raised by the use of methanol in the '963 patent are further increased by the use of the hot bath in which the flammable methanol is heated to close to its boiling point. Furthermore, heat transfer provided by the circulating methanol is inefficient and limits the rate at which the stator is cooled. The inefficiencies inherent in the circulating methanol also limit the responsiveness of the system to changing heat transfer requirements resulting in imprecision in the temperature control provided by the system.
The temperature control provided by the '963 system is further limited as a result of temperature variations necessarily created along the flow path of the circulating fluid. The circulation of a coolant fluid through a heat conveying member for the purposes of heat transfer between the member and the fluid inherently results in a variation in temperature along the path of the circulating coolant fluid as heat is added or removed from the coolant medium. Circulating systems of prior art cold cranking simulators, such as the simulator of U.S. Pat. No. 4,472,963 to Gyer, direct coolant fluid between an inlet located at a first side of the stator to an outlet located on an opposite side of the stator. The coolant fluid is directed in the conduits of the '963 simulator in a unidirectional circulation of the coolant fluid in which, at any given location of the stator, coolant is being directed in a single direction. As chilled methanol is directed about the stator, heat added to the methanol from the stator will raise the temperature of the methanol between the inlet and the outlet. As a result, temperature gradients will be created across the stator between the coolant inlet and outlet.
What is needed is a heat transfer system for varying the temperature of a test sample in a cold cranking simulator which provides for increased precision and uniformity in sample temperature control by increased responsiveness to changing heat transfer requirements and limitation of temperature gradients across the test sample.